Magnetic field detector for implantable medical devices

ABSTRACT

A torque sensor is described that detects the presence of an external magnetic field based on a torque imposed on a conductive coil of the sensor. The torque sensor includes a conductive coil forming a loop having one or more turns and a plurality of sensing elements adjacent to portions of the conductive coil. The sensing elements are configured to generate an output that changes as a function of a force imposed on the first sensing element by the respective portions of the conductive coil.

This application claims the benefit of U.S. Provisional Application No.61/639,159, filed on Apr. 27, 2012, the content of which is incorporatedherein by reference in its entirety.

TECHNICAL FIELD

The disclosure relates to sensors and techniques for detecting magneticfields, such as magnetic fields generated by magnetic resonance imaging(MRI) devices.

BACKGROUND

Magnetic resonance imaging (MRI) is a medical imaging technique used tovisualize detailed internal structures of a patient. A patient is placedat least partially within an MRI device during an MRI scan. The MRIdevice may generate a variety of magnetic and electromagnetic fields,including a static magnetic field (hereinafter “static MRI field”),gradient magnetic fields, and radio frequency (RF) fields. The staticMRI field may be generated by a primary magnet within the MRI device andmay be present prior to initiation of the MRI scan. The gradientmagnetic fields may be generated by electromagnets and may be presentduring the MRI scan. The RF magnetic fields may be generated bytransmitting/receiving coils and may be present during the MRI scan. Ifthe patient undergoing the MRI scan has an implantable medical device(IMD), the various fields produced by the MRI device may haveundesirable effects on the IMD.

SUMMARY

To reduce the effects that the various fields produced during an MRIscan have on the IMD, some IMDs may be programmed to an MRI-compatiblemode of operation (also referred to herein as an MRI operating mode orMRI mode) during the MRI scan. Typically, a clinician programs the IMDusing a programming device at some point in time prior to a scheduledMRI scan. After the patient receives the MRI scan, the clinician mayreprogram the IMD back to normal settings. The reprogramming processundertaken prior to, and after, scanning a patient with an IMD may beinconvenient to both the patient and the clinician. In some scenarios, apatient having an IMD may require an emergency MRI scan. Such scenariosmay not provide an adequate window of time around the MRI scan to allowfor reprogramming of the IMD.

An IMD according to the present disclosure may automatically detect thepresence of an MRI device (e.g., by detection of the static MRI field)prior to initiation of an MRI scan. For example, the IMD may detect theMRI device based on one or both of a strength of the magnetic fieldand/or a torque caused the magnetic field on a torque sensor.Furthermore, the IMD may differentiate the static MRI field from othermagnetic fields, such as magnetic fields generated by handheld magneticdevices, including telemetry head magnets, thus improving thespecificity with which the IMD identifies the source of a detectedmagnetic field based at least in part on the torque imposed by themagnetic field.

In response to detection of the static MRI field, the IMD may transitionfrom a normal operating mode to an MRI operating mode prior toinitiation of the MRI scan. While operating in the MRI mode, the IMD maybe configured such that it is less susceptible to being adverselyaffected by the gradient and RF fields emitted by the MRI device. Thecapability of the IMD to automatically detect the MRI device andtransition to the MRI mode may eliminate the need for manualreprogramming of the IMD prior to the MRI scan, or provide a failsafereprogramming mode in the event manual reprogramming is not undertaken.

In one example, this disclosure is directed to a sensor that includes aconductive coil forming a loop having one or more turns, a first sensingelement adjacent to a first portion of the conductive coil andconfigured to generate an output that changes as a function of a forceimposed on the first sensing element by the first portion of theconductive coil, and a second sensing element adjacent to the firstportion of the conductive coil and configured to generate an output thatchanges as a function of a force imposed on the second sensing elementby the first portion of the conductive coil. The first portion of theconductive coil is located between the first sensing element and thesecond sensing element. The sensor also includes a third sensing elementadjacent to a second portion of the conductive coil and configured togenerate an output that changes as a function of a force imposed on thethird sensing element by the second portion of the conductive coil and afourth sensing element adjacent to the second portion of the conductivecoil and configured to generate an output that changes as a function ofa force imposed on the fourth sensing element by the second portion ofthe conductive coil. The second portion of the conductive coil islocated on the opposite side of the loop as the first portion of theconductive coil. Also, the second portion of the conductive coil islocated between the third sensing element and the fourth sensingelement.

In another example, this disclosure is directed to an implantablemedical device comprising a torque sensor and a control moduleconfigured to analyze the output of the torque sensor to detect thepresence of an external magnetic field and control operation of theimplantable medical device based on the analysis. The torque sensorincludes a conductive coil forming a loop having one or more turns, afirst sensing element adjacent to a first portion of the conductive coiland configured to generate an output that changes as a function of aforce imposed on the first sensing element by the first portion of theconductive coil, and a second sensing element adjacent to the firstportion of the conductive coil and configured to generate an output thatchanges as a function of a force imposed on the second sensing elementby the first portion of the conductive coil. The first portion of theconductive coil is located between the first sensing element and thesecond sensing element. The sensor also includes a third sensing elementadjacent to a second portion of the conductive coil and configured togenerate an output that changes as a function of a force imposed on thethird sensing element by the second portion of the conductive coil and afourth sensing element adjacent to the second portion of the conductivecoil and configured to generate an output that changes as a function ofa force imposed on the fourth sensing element by the second portion ofthe conductive coil. The second portion of the conductive coil islocated on the opposite side of the loop as the first portion of theconductive coil. Also, the second portion of the conductive coil islocated between the third sensing element and the fourth sensingelement.

This summary is intended to provide an overview of the subject matterdescribed in this disclosure. It is not intended to provide an exclusiveor exhaustive explanation of the techniques as described in detailwithin the accompanying drawings and description below. Further detailsof one or more examples are set forth in the accompanying drawings andthe description below. Other features, objects, and advantages will beapparent from the description and drawings, and from the statementsprovided below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a conceptual diagram illustrating a magnetic resonance imaging(MRI) environment that includes an MRI device.

FIG. 2 is a conceptual diagram of an example implantable medical system.

FIG. 3 shows a schematic view of illustrating components of an IMD.

FIGS. 4A and 4B illustrate an example magnetic field torque sensor.

FIGS. 5A and 5B illustrate another example magnetic field torque sensor.

FIG. 6 is a block diagram that illustrates an example control module ofan IMD in further detail.

FIG. 7 is a flow diagram illustrating an example method of operation ofan IMD including a torque sensor in accordance with this disclosure.

FIG. 8 is a flow diagram illustrating an example method of operation ofan IMD in accordance with this disclosure.

FIG. 9 is a flow diagram illustrating another example method ofoperation of an IMD in accordance with this disclosure.

DETAILED DESCRIPTION

FIG. 1 is a conceptual diagram illustrating a magnetic resonance imaging(MRI) environment 10 that includes an MRI device 16. MRI device 16 mayinclude a patient table on which patient 12 is placed prior to andduring an MRI scan. The patient table is adjusted to position at least aportion of patient 12 within a bore of MRI device 16 (the “MRI bore”).While positioned within the MRI bore, the portion of patient 12 beingscanned is subjected to a number of magnetic and RF fields to produceimages of body structures for diagnosing injuries, diseases, and/ordisorders.

MRI device 16 includes a scanning portion that houses a primary magnetof MRI device 16 that generates a static MRI field. The static MRI fieldis a large non time-varying magnetic field that is typically alwayspresent around MRI device 16 whether or not an MRI procedure is inprogress. MRI device 16 also includes a plurality of gradient magneticfield coils that generate gradient magnetic fields. Gradient magneticfields are pulsed magnetic fields that are typically only present whilethe MRI procedure is in progress. MRI device further includes one ormore RF coils that generate RF fields. RF fields are pulsed highfrequency fields that are also typically only present while the MRIprocedure is in progress. Although the structure of MRI devices mayvary, it is contemplated that the techniques used herein to detect thestatic MRI field, which is generally applicable to a variety of otherMRI device configurations, such as open-sided MRI devices or otherconfigurations.

The magnitude, frequency or other characteristic of the static MRIfield, gradient magnetic fields and RF fields may vary based on the typeof MRI device 16 producing the field or the type of MRI procedure beingperformed. A 1.5 T MRI device, for example, will produce a staticmagnetic field of approximately 1.5 Tesla and have a corresponding RFfrequency of approximately 64 megahertz (MHz) while a 3.0 T MRI devicewill produce a static magnetic field of approximately 3.0 Tesla and havea corresponding RF frequency of approximately 128 MHz. However, otherMRI devices may generate different fields that may be detected inaccordance with the techniques of this disclosure.

Patient 12 is implanted with an implantable medical system 14. In oneexample, implantable medical system 14 may include an IMD connected toone or more leads. The IMD may be an implantable cardiac device thatsenses electrical activity of a heart of patient 12 and/or provideselectrical stimulation therapy to the heart of patient 12. For example,the IMD may be an implantable pacemaker, implantable cardioverterdefibrillator (ICD), cardiac resynchronization therapy defibrillator(CRT-D), cardioverter device, or combinations thereof. The IMD mayalternatively be a non-cardiac implantable device, such as animplantable neurostimulator or other device that provides electricalstimulation therapy or other therapy such as drug delivery.

Some or all of the various types of fields produced by MRI device 16 mayhave undesirable effects on implantable medical system 14. In oneexample, the gradient magnetic fields and/or the RF fields generatedduring the MRI procedure may induce energy on the conductors of theleads (e.g., in the form of a current). The induced energy on the leadsmay be conducted to the IMD and inappropriately detected asphysiological signals, a phenomenon often referred to as oversensing.The detection of the induced energy on the leads as physiologicalsignals may result in the IMD delivering therapy when it is not desired(e.g., triggering a pacing pulse) or withholding therapy when it isdesired (e.g., inhibiting a pacing pulse).

Upon detecting the presence of MRI device 16, the IMD is configured tooperate in an MRI operating mode or MRI mode. Operation of the IMD inthe “MRI mode” may refer to an operating state of the IMD that it isless susceptible to being adversely affected by the gradient magneticfields and RF fields emitted by MRI device 16 than the “normal mode” ofoperation. As such, operation of the IMD in the MRI mode may reduce, andpossibly eliminate, the undesirable effects that may be caused by thegradient magnetic fields and RF fields of MRI device 16. When operatingin the MRI mode, the IMD is configured to operate with differentfunctionality compared to the “normal mode” of operation. In oneexample, the IMD may operate in either a non-pacing mode (e.g., sensingonly mode) or in an asynchronous pacing mode while operating in the MRImode. The IMD may also turn off high voltage therapy (e.g.,defibrillation therapy) while operating in the MRI mode. The IMD mayalso turn off telemetry functionality, e.g., wakeup or other telemetryactivity, during operation in the MRI mode. In some examples, the MRImode may use other sensors (e.g., a pressure or acceleration sensor),different sense circuitry, or different sense algorithms to moreaccurately detect cardiac activity of the patient. Other adjustments maybe made as described herein. In this manner, patient 12 having implantedmedical system 14 may receive an MRI procedure with a reduced likelihoodof interference with operation of the IMD.

The IMD may transition to the MRI mode automatically in response todetecting MRI device 16. In accordance with the techniques of thisdisclosure, the IMD may include a magnetic field torque sensorconfigured to detect the presence of the static MRI field generated bythe primary magnet of MRI device 16. Details of example magnetic fieldtorque sensors will be described herein. In some instances, the IMD maydetect the presence of the static MRI field based the magnitude of themagnetic field as well as the torque.

After the MRI procedure is complete, the IMD may transition back to thenormal mode of operation, e.g., turn high voltage therapy back on and/orhave pacing that is triggered and/or inhibited as a function of sensedsignals. The IMD may automatically revert to the normal mode ofoperation in response to no longer detecting the presence of MRI device16, after expiration of a timer, or in response to some other predefinedcriteria, or a combination thereof. Alternatively, the IMD may bemanually programmed into the normal mode of operation via a commandreceived from an external device, such as programming device, viawireless telemetry.

FIG. 2 is a conceptual diagram of an example implantable medical system20, which may correspond with implantable medical system 14 of FIG. 1,in further detail. Implantable medical system 20 is also illustrated inconjunction with a programmer 22 and telemetry head 24. Implantablemedical system 20 includes an IMD 26 connected to leads 28 and 30.

IMD 26 may provide electrical stimulation to heart 32 via leads 28 and30. For example, IMD 26 may be an implantable pacemaker, implantablecardioverter defibrillator (ICD), cardiac resynchronization therapydefibrillator (CRT-D), cardioverter device, or combinations thereof. IMD26 includes a housing 34 and a connector block 36. Housing 34 andconnector block 36 may form a hermetic seal that protects components ofIMD 26. In some examples, housing 34 may comprise a metal or otherbiocompatible enclosure having separate halves. Connecter block 36 mayinclude electrical feedthroughs, through which electrical connectionsare made between conductors within leads 28 and 30 and electroniccomponents included within housing 34. As will be described in furtherdetail herein, housing 34 may house one or more processors, memories,transmitters, receivers, sensors, sensing circuitry, therapy circuitryand other appropriate components. Housing 34 is configured to beimplanted in a patient, such as patient 12.

Leads 28 and 30 each include one or more electrodes. In the exampleillustrated in FIG. 2, leads 28 and 30 each include a respective tipelectrodes 38 and 40 and ring electrodes 42 and 44 located near a distalend of their respective leads 28 and 30. When implanted, tip electrodes38 and 40 and/or ring electrodes 42 and 44 are placed relative to or ina selected tissue, muscle, nerve or other location within the patient12. In the example illustrated in FIG. 2, tip electrodes 38 and 40 areextendable helically shaped electrodes to facilitate fixation of thedistal end of leads 28 and 30 to the target location within patient 12.In this manner, tip electrodes 38 and 40 are formed to define a fixationmechanism. In other embodiments, one or both of tip electrodes 38 and 40may be formed to define fixation mechanisms of other structures. Inother instances, leads 28 and 30 may include a fixation mechanismseparate from tip electrode 38 and 40. Fixation mechanisms can be anyappropriate type, including a grapple mechanism, a helical or screwmechanism, a drug-coated connection mechanism in which the drug(s)serves to reduce infection and/or swelling of the tissue, or otherattachment mechanism.

One or more conductors (not shown in FIG. 2) extend within leads 28 and30 from connector block 36 along the length of the lead to engagerespective tip electrodes 38 and 40 and ring electrode 42 and 44. Inthis manner, each of electrodes 38, 40, 42 and 44 is electricallycoupled to a respective conductor within its associated lead body. Forexample, a first electrical conductor can extend along the length of thebody of lead 28 from connector block 36 and electrically couple to tipelectrode 38 and a second electrical conductor can extend along thelength of the body of lead 28 from connector block 36 and electricallycouple to ring electrode 42. The respective conductors may electricallycouple to circuitry, such as a therapy module or a sensing module, ofIMD 26 via connections in connector block 36. The electrical conductorstransmit therapy from a therapy module within IMD 26 to one or more ofelectrodes 38, 40, 42, and 44 and transmit sensed electrical signalsfrom one or more of electrodes 38, 40, 42, and 44 to the sensing modulewithin IMD 26.

IMD 26 may communicate with programmer 22 using any of a variety ofwireless communication techniques known in the art. Examples ofcommunication techniques may include, for example, low frequencyinductive telemetry or RF telemetry, although other techniques are alsocontemplated. Programmer 22 may be a handheld computing device, desktopcomputing device, a networked computing device, or other computingdevice configured to communicate with IMD 26. Programmer 22 may includea non-transitory computer-readable storage medium having instructionsthat, when executed, cause a processor of programmer 22 to provide thefunctions attributed to programmer 22 in the present disclosure.

Programmer 22 retrieves data from IMD 26. Data retrieved from IMD 26using programmer 22 may include cardiac EGMs stored by IMD 26 thatindicate electrical activity of heart 32. Data may also include markerchannel data that indicates the occurrence and timing of sensing,diagnosis, and therapy events associated with IMD 26. Additionally, datamay include information regarding the performance or integrity of IMD 26or other components of implantable medical system 20, such as leads 28and 30, or a power source of IMD 26. Programmer 22 may also transferdata to IMD 26. Data transferred to IMD 26 using programmer 22 mayinclude, for example, values for operational parameters, electrodeselections used to deliver electrical stimulation, waveform selectionsused for electrical stimulation, configuration parameters for detectionalgorithms, or the other data. Although not illustrated in FIG. 2, IMD26 may communicate with other devices not implanted within patient 12,such as a patient monitor.

Programmer 22 may, in one example, communicate with IMD 26 via atelemetry head 24. Telemetry head 24 may include a telemetry head magnet46. Telemetry head magnet 46 generates a magnetic field (“telemetry headfield”). IMD 26 may detect the presence of telemetry head magnet 46(e.g., by detecting the telemetry head field) and may operate in atelemetry head mode in response to detection of telemetry head magnet46. Operation of IMD 26 in the “telemetry head mode” may describe atypical operating state of IMD 26 in response to detection of telemetryhead magnet 46, and may be different from the MRI mode and the normalmode. For example, after IMD 26 detects telemetry head magnet 46, IMD 26may enter the telemetry head mode and may communicate with programmer122 or other external device by wireless telemetry via telemetry head 24or RF telemetry or other telemetry technique, to transfer data toprogrammer 22 and/or receive data from programmer 22. IMD 26 may alsodisable tachycardia detection when operating in the telemetry head mode,but may still keep sensing functionality enabled.

In some examples, telemetry head magnet 46 may include a permanentmagnet. The permanent magnet may have an area that is approximatelyequal to the area of IMD 26 so that when telemetry head 24 is placedover top of IMD 26, the permanent magnet may substantially cover IMD 26.In some examples, telemetry head magnet 46 may include handheld magneticdevices other than a permanent magnet, such as an electromagnet thatgenerates the telemetry head field.

As described above with respect to FIG. 1, IMD 26 also operates in theMRI mode in response to detecting the static magnetic field associatedwith MRI device 16. As such, IMD 26 may operate in different operatingmodes in response to detecting magnetic fields from different sources,e.g., operate in the MRI mode in response to detecting the static MRIfield and operate in the telemetry head mode in response to detectingthe telemetry head field. To this end, IMD 26 may be configured todifferentiate between magnetic fields from the different sources basedon characteristics associated with the magnetic fields.

Typically, the strength (or magnitude) of the static magnetic fieldassociated with MRI device 16 is much larger than the strength (ormagnitude) of the telemetry head magnet 46 or other magnetic fieldspatient 12 encounters. MRI device 16 may have a static magnetic fieldthat has a magnitude that is larger than approximately 0.5 Tesla. Thestrength of telemetry head magnet 46, on the other hand, is typically inthe millitesla (mT) range. For example, telemetry head magnet 46 mayhave a magnitude in the range of approximately 10 mT to 100 mT. Inaccordance with the techniques of this disclosure, IMD 26 may include amagnetic field torque sensor to distinguish the telemetry head field (orother magnetic fields typically encountered by patient 12) from thestatic MRI field based on output from the magnetic field torque sensor.

Additionally, other devices that generate magnetic fields similar totelemetry head magnet 46 may come in proximity to IMD 26. Such devicesmay include, but are not limited to, permanent magnets andelectromagnets other than the patient magnet. Telemetry head magnet 46may, therefore, generally represent any magnetic device (e.g., handheldmagnetic device) or other magnetic field source that generates amagnetic field similar to that of telemetry head magnet 46. In general,most “environmental” magnetic field sources, such as welders, electricmotors, and theft detection gates, to name a few, will exhibit amagnetic field similar to that of telemetry head magnet 46, while fewmagnetic field sources may exhibit a magnetic field in scale as large asthe permanent magnet of MRI device 16.

Although IMD 26 is illustrated as an implantable cardiac stimulationdevice (e.g., a pacemaker, ICD, CRT-D, or the like), in other examples,an implantable device that detects the static MRI field and operates inthe MRI mode according to the present disclosure may include animplantable drug pump or an implantable neurostimulator that provides atleast one of deep brain stimulation, vagus nerve stimulation, gastricstimulation, pelvic floor stimulation, spinal cord stimulation, or otherstimulation. In other examples, an implantable device that detects thestatic MRI field and operates in the MRI mode may include any otheractive implantable medical device that includes electronics that thefields produced by MRI device 16 may interfere with. In other examples,a device that detects the static MRI field and operates in the MRI modemay include an external device.

FIG. 3 shows a schematic view of illustrating components of IMD 26within housing 34. Housing 34 defines a cavity 50 in which components ofIMD 26 are housed. IMD 26 includes a power source 52 housed withincavity 50. Power source 52 may include a battery, e.g., a rechargeableor non-rechargeable battery. IMD 26 may also include a printed circuitboard (PCB) 54 that includes electronic components of IMD 26, which inthe example of FIG. 3 include, but are not limited to, a control module56, magnetic field torque sensor(s) 58, and magnetic field strengthsensor 60.

PCB 54 may not be limited to typical PCB structures, but may insteadrepresent any structure within IMD 26 that is used to mechanicallysupport and electrically connect control module 56, magnetic fieldtorque sensor(s) 58, magnetic field strength sensor 60, power source 52,and other electronic components within housing 34. In some examples, PCB54 may include one or more layers of conductive traces and conductivevias that provide electrical connection between control module 56,magnetic field torque sensor(s) 58, and magnetic field strength sensor60 as well as and electrical connection between power source 52 andcontrol module 56, magnetic field torque sensor(s) 58, and magneticfield strength sensor 60 such that power source 52 may provide thosecomponents. Conductors within leads 28 and 30 may be connected tocontrol module 56 on PCB 54 through connecting wires 62. For example,connecting wires 62 may be connected to conductors within leads 28 and30 at one end (e.g., via one or more feed throughs), and connected toPCB connection points 64 on PCB 54 at the other end.

Although the electronic components of IMD 26 are illustrated as includedon a single PCB, it is contemplated that the electronic componentsdescribed herein may be included elsewhere within IMD 26, e.g., on othersupporting structures within IMD 26, such as additional PCBs (notshown). In other examples, electronic components within IMD 26 may bemounted to the inside of housing 34 within cavity 50 or mounted to theoutside of housing 34 and connected to components on the inside ofhousing 34 through a feed through (not shown) in housing 34. In stillother examples, electronic components may be mounted on or withinconnector block 36 or connected to one or more of leads 28 and 30.

Control module 56, and modules included within control module 56,represents functionality that may be included in IMD 26 of the presentdisclosure. Modules of the present disclosure may include any discreteand/or integrated electronic circuit components that implement analogand/or digital circuits capable of producing the functions attributed tothe modules herein. For example, the modules may include analogcircuits, e.g., amplification circuits, filtering circuits, and/or othersignal conditioning circuits. The modules may also include digitalcircuits, e.g., combinational or sequential logic circuits, memorydevices, etc. The memory may be any non-transitory computer-readablestorage medium, including any volatile, non-volatile, magnetic, orelectrical media, such as a random access memory (RAM), read-only memory(ROM), non-volatile RAM (NVRAM), electrically-erasable programmable ROM(EEPROM), Flash memory, or any other memory device. Furthermore, thememory may include instructions that, when executed by one or moreprocessing circuits, cause the modules to perform various functionsattributed to the modules herein.

The functions attributed to the modules herein may be embodied as one ormore processors, hardware, firmware, software, or any combinationthereof. Depiction of different features as modules is intended tohighlight different functional aspects and does not necessarily implythat such modules must be realized by separate hardware or softwarecomponents. Rather, functionality associated with one or more modulesmay be performed by separate hardware or software components, orintegrated within common or separate hardware or software components.

Field strength sensor 60 generates signals that vary as a function ofthe strength of the magnetic field. Field strength sensor 60 may, forexample, generate and output a voltage signal that varies as a functionof the strength of the magnetic field. In another example, fieldstrength sensor 60 may only output a signal when a magnetic fieldexceeds a threshold field strength, as is the case for a Reed switch orother magnetic switch that closes in response to being exposed to amagnetic field that exceeds a minimum amplitude or strength. Fieldstrength sensor 60 may, for example, be one or more types of magneticfield sensors that may include, but are not limited to, Hall-effectsensors, giant magnetoresistance (GMR) based sensors, anisotropicmagnetoresistance (AMR) based sensors, tunneling magnetoresistance (TMR)based sensors, or any other type of magnetic field sensor suitable formeasuring a magnitude of a magnetic field to which it is exposed.

Magnetic field torque sensor(s) 58 generates signals that vary as afunction of a torque exerted on sensor(s) 58 by an external magneticfield. FIGS. 4A and 4B illustrate one example magnetic field torquesensor 58. Magnetic field torque sensor 58 includes a coil 66constructed of a conductive material having one or more turns and forcesensors 68A-D (collectively referred to herein as “force sensors 68”)within a housing 69. Coil 66 may be constructed of wire, metal,conductive trace, or other conductor or conductive material. In theexample of FIGS. 4A and 4B, coil 66 is formed into a squareconfiguration having a single turn forming a plane 70. However, coil 66may be formed into other configurations, e.g., rectangle, oval, circleor other shape. Additionally, coil 66 may be formed to have more thanone turn of the conductive material. For example, coil 66 may be formedto have a plurality of turns formed in a single plane, e.g., in a spiralshape. In other examples, coil 66 may be formed to have a plurality ofturns formed in a multiple planes, e.g., coil 66 being wound such thateach turn is located on top of the previous turn. Other configurationsare also contemplated.

A current is supplied to coil 66 by one of the components of IMD 26. Theflow of electric current through coil 66 produces a magnetic field. Themagnetic field produced by the current supplied to coil 66 will bereferred to herein as the “internal magnetic field.” As such, coil 66 ofmagnetic field torque sensor 58 functions as a small electromagnet. Whenpatient 10 and IMD 26 are subjected to a magnetic field generated by anexternal source (referred to herein as the “external magnetic field”),such as the primary magnet of MRI device 16, the external magnetic fieldand the internal magnetic field interact such that a magnetic moment ofcoil 66 attempts to align with the external magnetic field. Theinteraction of the internal magnetic field and the external magneticfield imposes a torque on coil 66. The torque (T) exerted on a currentloop, e.g., defined by coil 66, is given by:

T=μ×B,  (1)

where μ is the magnetic moment of coil 66, and B is the externalmagnetic field. The torque (T), magnetic moment (μ), and the externalmagnetic field (B) are all vector quantities. The magnitude of themagnetic moment (μ) is equal to:

μ=N·I·A  (2)

where I is the current through coil 66, A is the area of the loop formedby coil 66, and N is equal to the number of turns of coil 66. Thedirection of the magnetic moment of coil 66 is determined by the vectorcross product. In the example illustrated in FIG. 4A, the vectordirection of the magnetic moment of coil 66 is along the positivez-axis. The external magnetic field (B) may be defined as:

B=B ₁ {circumflex over (x)}+B ₂ ŷ+B ₃ {circumflex over (z)}  (3)

where B₁ is the magnitude of the vector component of the externalmagnetic field in the x-direction ({circumflex over (x)}), B₂ is thevector component of the external magnetic field in the y-direction (ŷ),and B₃ is the vector component of the external magnetic field in thez-direction ({circumflex over (z)}).

The torque exerted on coil 66 produces forces on some of force sensors68. Force sensors 68 are configured to measure the force in thedirection of rotation and output a signal representative of the force.In the example illustrated in FIGS. 4A and 4B, the interaction of theinternal and external magnetic fields creates a torque (represented asarrow “T” in FIGS. 4A and 4B) having an axis of rotation around they-axis, which is orthogonal to the direction of the dipole moment ofcoil 66. The forces (represented as arrows “F1” and “F2” in FIGS. 4A and4B) created by the torque act on opposing sides of coil 66 and inopposing directions.

Force sensors 68 are arranged adjacent to portions of coil 66 to measurethe force imposed by the torque on the respective portions of coil 66.Force sensor 68A is arranged adjacent to a first portion of coil 66extending along the y-axis and force sensor 68B is arranged adjacent toan opposite side of the first portion of coil 66. In other words, forcesensor 68A and force sensor 68B may be viewed as sandwiching the firstportion of coil 66, i.e., the first portion of coil 66 is locatedbetween force sensors 68A and 68B. Force sensor 68C and 68D aresimilarly arranged adjacent to opposite sides of a second portion ofcoil 66 extending along the y-axis, such that the second portion of coil66 is located (or sandwiched) between force sensors 68C and 68D. Thefirst portion of coil 66 and the second portion of coil 66 are locatedon opposite sides of the loop. In the example torque sensor illustratedin FIGS. 4A and 4B, force sensors 68A and 68C are located in a firstplane 72 that is substantially parallel to plane 70 defined by coil 66and force sensors 68B and 68D are located in a second plane 74 that issubstantially parallel to plane 70 defined by coil 66.

As indicated above, force sensors 68 are configured to measure a forceexerted on sensors 68 at their respective locations along coil 66 by thetorque on coil 66 caused by the interaction of the internal and externalmagnetic fields. Force sensors 68 generate signals representative of theforce measured at their respective locations. In one example, forcesensors 68 may output a voltage that varies as a function of the forceexerted on the respective sensors 68. As coil 66 is subjected to theexternal magnetic field, the torque on coil 66 creates forces on some orall of force sensors 68, thereby changing the output (e.g., voltage)generated by sensors 68. In one example, force sensors 68 may bemechanically coupled to coil 66 such that the torque results in anincreased pressure on some or all of force sensors 68 in the directionof rotation. In other examples, force sensors 68 may not be mechanicallycoupled to coil 66, but instead arranged so that sensors 68 areimmediately adjacent to the respective portions of coil 66 and anyphysical displacement of coil 66 due to the torque exerted by theinteraction of the internal and external magnetic fields initiatescontact with force sensors 68.

Force sensor 68 measure the force exerted along the axis correspondingto the direction of the magnetic moment of the coil 66. In the exampleof FIGS. 4A and 4B, force sensors 68 measure the force exerted along theZ-axis, in either the positive and negative direction. The torque oncoil 66 generates a force F1 in the positive Z-direction on force sensor68A and a force F2 in the negative Z-direction on force sensor 68D.Force sensors 68B and 68C measure little, if any, force since the torqueon coil 66 is away from sensors 68B and 68C. When the magnetic fieldtorque sensor 58 is exposed to a magnetic field that causes a torque inthe opposite direction illustrated in FIGS. 4A and 4B the forces causedby rotation of coil 66 would be exerted on force sensors 68B and 68C andlittle, if any, force would be exerted on force sensors 68A and 68D.Magnetic field torque sensor 58 outputs signals that vary as a functionof the force exerted on each of the force sensors 68. In some instances,a stronger external magnetic field, such as that produced by MRI device16, generates a larger force on force sensors 68 than a smaller externalmagnetic field, such as that produced by telemetry head magnet 46,assuming that the magnetic field orientation is substantially the same.As will be described in further detail herein, control module 56analyzes the signals output by magnetic field torque sensor 58 todetermine whether IMD 26 is exposed to an external magnetic field, suchas the static magnetic field generated by the primary magnet of an MRIdevice.

In one example, each of force sensors 68 may be a strip of piezoelectricfilm that generates an electrical signal (e.g., charge or voltage) inresponse to a change in the physical geometry, e.g., stretching, bendingor other physical change, caused by the pressure or force exerted by thetorque of coil 66. Strips of piezoelectric film may, in some instances,require no external power in order to function, are lightweight, thin,and flexible. Additionally, strips of piezoelectric film are also verysensitive, making them suitable for detecting very low-level mechanicalsignals. In other examples, however, force sensors 68 may include othertypes of sensors or combinations of sensors, such as sensors thatinclude a membrane or transducer element to detect physical displacementcaused by the torque on coil 66, including but not limited to MEMSsensors, optical sensors, mechanical resonance sensors, piezo resistiveelements, or the like. Force sensors 68 may, in some instances, beelectrically isolated from coil 66 via a dielectric material 76. Inother instances, coil 66 may be conductor with an outer insulation layerthat electrically isolates coil 66 form force sensors 68.

In some instances, additional force sensors 68 may be placed elsewherealong coil 66. In the example of FIGS. 4A and 4B, additional forcesensors 68 may be placed along the portions of loop that extend in thex-direction. For example, two force sensors 68 may be arranged adjacentto opposite sides of a third portion of coil 66 extending along thex-axis such that the third portion of coil 66 along the x-axis islocated (or sandwiched) between the two force sensors and two forcesensors may be arranged adjacent to opposite sides of a fourth portionof coil 66 extending along the x-axis such that the fourth portion ofcoil 66 along the x-axis is located (or sandwiched) between the twoforce sensors. The third portion of coil 66 and the fourth portion ofcoil 66 are located on opposite sides of the loop.

Coil 66 and force sensors 68 of torque sensor 58 of FIGS. 4A and 4B arearranged in a single detection axis. Coil 66 and force sensors 68 arearranged in the x-y plane to measure the forces imposed by a torque oncoil 66 having an axis of rotation that is not along the z-axis. Themagnitude of the torque exerted on coil 66 (T₁) along the axiscorresponding to the direction of the dipole moment of coil 66 (e.g.,the z-axis in the example of FIGS. 4A and 4B) is equal to:

T ₁=μ₃(B ₂ {circumflex over (x)}−B ₁ ŷ)  (4)

where μ₁ is equal to the magnitude of the magnetic moment defined byequation (1). As such, torque sensor 58 having single coil 66 can onlydetect forces caused by a torque in two dimensions.

In instances in which torque sensor 58 is configured to detect theforces imposed by a torque in two directions, IMD 26 may include twotorque sensors 58 physically arranged in different planes within IMD 26such that the first torque sensor 58 is in a plane that is not alignedwith a plane in which the second torque sensor 58 is located. In oneexample, the plane of the first torque sensor 58 and the plane of thesecond torque sensor 58 may be orthogonal to one another. However, inother instances, the plane of the first torque sensor 58 and the planeof the second torque sensor 58 may be orthogonal as long as they are notaligned. For purposes of illustration, however, the first and secondtorque sensors 58 will be described herein as being arranged orthogonalto one another. The second torque sensor 58 is substantially similar tothe first torque sensor having a second coil 66 and force sensors 68arranged within a housing 69 as described above with respect to FIGS. 4Aand 4B, but physically arranged such that the dipole moment of thesecond torque sensor is in a direction orthogonal to the dipole momentof the first torque sensor. For example, if the direction of the dipolemoment of the first torque sensor is along the z-axis (as illustrated inFIGS. 4A and 4B), then the dipole moment of the second torque sensorwould be either along the x-axis or the y-axis. The magnitude of thetorque (T₂) exerted on a coil 66 of a second torque sensor 58 having itsdipole moment along the x-axis is equal to:

T ₂=μ₁(B ₂ {circumflex over (z)}−B ₃ ŷ)  (5)

where μ₂ is equal to the magnitude of the magnetic moment of coil 66 ofthe second torque sensor 58 defined by equation (1).

Utilizing two torque sensors arranged orthogonal to one another (or atleast arranged such that they are not in parallel planes) results in atleast one of the torque values being nonzero for a magnetic field havingany orientation. As such, the arrangement of two torque sensors in sucha manner provides IMD 26 the ability to detect a magnetic field with anyorientation.

FIGS. 5A and 5B illustrate views of another example torque sensor 58′.Torque sensor 58′ is substantially similar to torque sensor 58 of FIGS.4A and 4B, but includes a second coil 66′ and force sensors 68′ locatedin a second detection axis that is orthogonal to the first detectionaxis within the same housing 69. In the example of FIGS. 5A and 5B, thesecond coil 66′ and force sensors 68′ are arranged in the y-z plane. Thearrangement of coil 66′ and force sensors 68′ is substantially similarto that illustrated in FIGS. 4A and 4B and described in detail abovewith respect to coil 66 and force sensors 68.

Force sensor 68A′ is arranged adjacent to a first portion of coil 66′extending along the y-axis and force sensor 68B′ is arranged adjacent toan opposite side of the first portion of coil 66′. In other words, forcesensor 68A′ and force sensor 68B′ may be viewed as sandwiching the firstportion of coil 66′, i.e., the first portion of coil 66′ is locatedbetween force sensors 68A′ and 68B′. The arrangement also includes forcesensors 68C′ and 68D′ arranged adjacent to opposite sides of a secondportion of coil 66′ extending along the y-axis, such that the secondportion of coil 66′ is located (or sandwiched) between force sensors68C′ and 68D′. The first portion of coil 66′ and the second portion ofcoil 66′ are located on opposite sides of the loop.

The torques on coil 66 and coil 66′ are defined by equations (4) and (5)above, respectively. As described above, arranging coils 66 and 66′ suchthat they are substantially orthogonal, torque sensor 58′ may detect amagnetic field having any orientation. Again, however, coils 66 and 66′need not be orthogonal but should be arranged such that they are notaligned in the same plane or parallel planes.

FIG. 6 is a block diagram that illustrates an example control module 56of IMD 26 in further detail. Control module 56 includes a processingmodule 80, memory 82, therapy module 84, sensing module 86,communication module 88, and field discrimination module 89.

Processing module 80 may communicate with memory 82. Memory 82 mayinclude computer-readable instructions that, when executed by processingmodule 80 or other component of IMD 26, cause processing module 80 orother component of IMD 26 to perform the various functions attributed tothem herein. Memory 82 may be any non-transitory computer-readablestorage medium, including any volatile, non-volatile, magnetic, orelectrical media, such as RAM, ROM, NVRAM, EEPROM, Flash memory, or anyother digital media.

Processing module 80 may also communicate with therapy module 84 andsensing module 86. Therapy module 84 and sensing module 86 areelectrically coupled to electrodes 38, 40, 42, and 44 of leads 28 and30. Sensing module 86 is configured to analyze signals from electrodes38, 40, 42, and 44 of leads 28 and 30 in order to monitor electricalactivity of heart 32, such as the depolarization and repolarization ofheart 32. Processing module 80 may detect cardiac activity based onsignals received from electrical sensing module 80. In some examples,processing module 80 may detect tachyarrhythmias based on signalsreceived from sensing module 86, e.g., using any suitabletachyarrhythmia detection algorithm.

Processing module 80 may generate EGM waveforms based on the detectedcardiac activity. Processing module 80 may also generate marker channeldata based on the detected cardiac activity. For example, marker channeldata may include data that indicates the occurrence and timing ofsensing, diagnosis, and therapy events associated with IMD 26.Additionally, marker channel data may include information regarding theperformance or integrity of components of IMD 26 or leads 28 and 30.Processing module 80 may store EGM waveforms and marker channel data inmemory 82. Processing module 80 may later retrieve stored EGMs frommemory 82, e.g., upon a request from programmer 22 via communicationmodule 88.

Therapy module 84 is configured to generate and deliver therapy, such aselectrical stimulation therapy, to heart 32 or other desired location.Processing module 80 may control therapy module 84 to deliver electricalstimulation therapy to heart 32 according to one or more therapyprograms, which may be stored in memory 82. For example, processingmodule 80 may control therapy module 84 to deliver pacing pulses toheart 32 based on one or more therapy programs and signals received fromsensing module 86.

Therapy module 84 may also be configured to generate and delivercardioversion and/or defibrillation shocks to heart 32 in addition to orinstead of pacing pulses. Processing module 80 may control therapymodule 84 to deliver the cardioversion and defibrillation pulses toheart 32. For example, in the event that processing module 80 detects anatrial or ventricular tachyarrhythmia, processing module 80 may load ananti-tachyarrhythmia pacing regimen from memory 82, and control therapymodule 84 to implement the anti-tachyarrhythmia pacing regimen. Therapymodule 84 may include a high voltage charge circuit and a high voltageoutput circuit when therapy module 84 is configured to generate anddeliver defibrillation pulses to heart 32, e.g., should the ATP therapynot be effective to eliminate the tachyarrhythmia.

Communication module 88 includes any suitable hardware, firmware,software or any combination thereof for communicating with anotherdevice, such as programmer 22 and/or a patient monitor, e.g., bywireless telemetry. Under the control of processing module 80,communication module 88 may receive downlink telemetry from and senduplink telemetry to programmer 22 and/or a patient monitor with the aidof an antenna (not shown) in IMD 26. Processing module 80 may providethe data to be uplinked to programmer 22 and the control signals for atelemetry circuitry within communication module 88.

Control module 56 obtains signals from torque sensors and field strengthsensor 60 and processes the signals to detect the presence of a magneticfield. In the example illustrated in FIG. 6, control module 56 mayreceive signals from torque sensor 58A and 58B (collectively “torquesensors 58”), which are two separate torque sensors physically arrangedsubstantially orthogonal to one another within IMD 26. Alternatively,control module 56 may receive signals from torque sensor 58′(illustrated as a dotted line representative of an alternativearrangement), which is described in detail with respect to FIGS. 5A and5B.

In some examples, IMD 26 may include additional sensors other thantorque sensors 58 and field strength sensor 60, with which sensingmodule 86, processing module 80 or field discrimination module 89 maycommunicate. For example, IMD 26 may include one or more of a motionsensor (e.g., an accelerometer or piezoelectric element), a heart soundsensor, or a pressure sensor (e.g., a capacitive sensor) that sensesintracardiac or other cardiovascular pressure. The one or moreadditional sensors may be located within housing 34, outside of housing34, attached to one or more of leads 28 or 30, or wirelessly coupled tocontrol module 56 via communication module 88. In some examples, torquesensors 58 or field strength sensor 60 may be located outside of housing34, attached to one or more of leads 28 or 30, or wirelessly coupled tocontrol module 56 via communication module 88.

Field discrimination module 89 is in electrical communication withtorque sensors 58, field strength sensor 60, and processing module 80.Field discrimination module 89 may include circuits that interface withtorque sensors 58 and field strength sensor 60. For example, fielddiscrimination module 89 may include circuits that provide current tocoils 66 of torque sensors 58. Field discrimination module 89 may alsoinclude amplification circuits, filtering circuits, and/or other signalconditioning circuits that process signals received from torque sensors58 and field strength sensor 60. In some examples, field discriminationmodule 89 may also include circuits that digitize the conditionedsignals and communicate the digitized signals to processing module 80.

Field discrimination module 89 receives signals from field strengthsensor 60 and determines the strength of the magnetic field. Fielddiscrimination module 89 also receives signals from torque sensors 58and determines whether a torque is exerted on coils 66 of the respectivesensor 58 by an external magnetic field. As described in detail herein,field discrimination module 89 may identify the source of the detectedmagnetic field as either the primary magnet of MRI device or telemetryhead magnet 46 based on the strength and/or the torques detected usingthe output of field strength sensor 60 and torque sensors 58,respectively.

In one example, field discrimination module 89 may obtain the signalsoutput by torque sensors 58 and determine whether IMD 26 is exposed toan external magnetic field based on the signals obtained from torquesensors 58. As described above, a current is supplied to coils 66 oftorque sensors 58 by one of the components of IMD 26, such as fielddiscrimination module 89, to produce the internal magnetic field. Whenpatient 10 and IMD 26 are subjected to an external magnetic field, theexternal magnetic field and the internal magnetic field interact byimposing a torque on coils 66 in an attempt to align a magnetic momentof coils 66 with the external magnetic field. Force sensors 68 of torquesensors 58 generate signals representative of the force imposed on themby the torque of coils 66.

Field discrimination module 89 may, for example, receive signalsrepresentative of the force imposed on each of force sensors 68 by thetorque of coils 66. The signals may, for instance, be voltage signals.Field discrimination module 89 may analyze the forces imposed on theforce sensors 68 to detect the presence of the external magnetic field.For instance, field discrimination module 89 may detect the presence ofthe external magnetic field when a force is imposed on a pair of forcesensors 68 on opposing sides of coil 66 and in opposing directions. Forexample, field discrimination module 89 may detect presence of theexternal magnetic field when forces that exceed a threshold are detectedon force sensors 68A and 68D at the same time or forces that exceed athreshold are detected on force sensors 68B and 68C at the same time.Detecting forces on force sensors 68 on opposing sides of coil 66 and inopposing directions distinguishes a force caused by torque versus aforce caused by translational motion. In some instances, fielddiscrimination module 89 may additionally require that the magnitude ofthe imposed force detected on force sensors 68 exceeds a magnitudethreshold, thus using the magnitude of the forces on sensors 68 as apossible discriminator between smaller external magnetic fields (e.g.,telemetry head fields) and large external magnetic fields (e.g., MRIstatic magnetic field). In some instances, torque sensors 58 may includememory and/or processing circuitry to process the signals of forcesensors 68 and output and indicator as to whether or not a torque isdetected.

The sensitivity of torque sensors 58 may be adjusted such that only thetorque caused by a large magnetic field, such as the primary magnet ofMRI device 16 is detected. The sensitivity of torque sensors 58 may, forexample, be adjusted by adjusting the number of turns of coils 66, thearea of the loop formed by coils 66, the amount of current supplied tocoils 66, the threshold torque value, or the like. For example,increasing the number of turns of coil 66 increases the sensitivity oftorque sensors 58. Likewise, the magnitude of the current supplied tocoil 66 may also affect the sensitivity of torque sensors 58. The morecurrent supplied to coil 66, the larger the internal magnetic field andthus the interaction with the external magnetic field. As such, thelarger the current supplied to coil 66, the more sensitive torquesensors 58 is. Additionally, the material used as force sensors 68 mayfurther affect the sensitivity. Using a stiffer material as forcesensors 68 require an increased torque to measure the same amount offorce. As such, stiffer material decreases the sensitivity of torquesensors 58. The magnitude threshold values utilized by fielddiscrimination module 89 may be selected to require more or less torqueon coil 66, this increasing or decreasing the sensitivity of torquesensors 58. One or more of these parameters may be adjusted or selectedto provide torque sensors 58 with the desired sensitivity.

By adjusting the sensitivity of torque sensors 58 such that it iscapable of detecting torque when IMD 26 is exposed to the primary magnetof MRI device 16, but not detect when IMD 26 is exposed to smallermagnetic fields, such as the magnetic field generated by telemetry headmagnet 46, torque sensors 58 may be utilized as an MRI detector. Inother instances, field discrimination module 89 may use the magnitude ofthe forces exerted on force sensors 68 to differentiate between IMD 26being exposed to the primary magnet of MRI device 16 or smaller magneticfields, such as the magnetic field generated by telemetry head magnet46. For example, field discrimination module 89 may compare the forcesexerted on force sensors 68 by coils 66 with respective threshold valuesand, detect presence of MRI device 16 when the magnitude the forces onone of the torque sensors exceeds the threshold values. However, ifforces are present, but the magnitude of the forces on neither of thetorque sensors exceeds the respective thresholds, field discriminationmodule 89 detects presence of telemetry head magnet 46.

Processing module 80 may transition IMD 26 from operation in the normalmode to operation in one of the telemetry head mode or the MRI mode,depending on the source of the magnetic field indicated by fielddiscrimination module 89. Processing module 80 may operate in the normalmode while no magnetic field is detected. While operating in the normalmode, processing module 80 may provide typical sensing, pacing, anddefibrillation functions without preparing for communication withtelemetry head 24 or preparing IMD 26 for entry into an MRI environment.Operation of processing module 80, however, may change whentransitioning IMD 26 from operation in the normal mode to operation ineither the telemetry head mode or the MRI mode.

Processing module 80 may transition IMD 26 from operation in the normalmode to operation in the telemetry head mode in response to indicationfrom field discrimination module 89 that the source of the magneticfield is telemetry head magnet 46. While in the telemetry head mode,processing module 80 may control communication module 88 to communicatewith programmer 22 via telemetry head 24, e.g., download data fromprogrammer 22 and upload data to programmer 22.

Processing module 80 may transition IMD 26 from operation in the normalmode to operation in the MRI mode in response to indication from fielddiscrimination module 89 that the source of the magnetic field is theprimary magnet of MRI device. While in the MRI mode, processing module80 may execute commands that prepare IMD 26 for exposure to an MRIenvironment. For example, processing module 80 may notify an operator,via communication module 88, that the MRI field has been detected andthat IMD 26 is configured for operation during an MRI scan. In otherexamples, processing module 80 may disable telemetry functionalityduring operation in the MRI mode. With respect to pacing functionality,processing module 80 may control therapy module 84 to operate in anasynchronous mode in which pacing may be provided according to a settiming, i.e., fixed, predetermined timing, and may not be responsive toevents sensed by sensing module 86 such as sensed cardiac P or R waves.In other examples, processing module 80 may control IMD 26 to operate ina sensing only mode in which no pacing therapy is provided. When therapymodule 84 includes defibrillator functionality, processing module 80 maydisable tachycardia detection and defibrillation in the MRI mode so thatany electrical noise induced in leads 28 or 30 may not be misinterpretedas a tachycardia event. Processing module 80 may also discontinuestoring EGM waveforms in memory 82 and may disable diagnostic functionssince the gradient and RF fields may corrupt the EGM waveforms. In someexamples, processing module 80 may use other sensors (e.g., a pressureor acceleration sensor), different sense circuitry, or different sensealgorithms to detect cardiac activity of the patient. In other examples,processing module 80 may instruct sensing module 86 to filter outsignals induced by the MRI fields. It is contemplated that processingmodule 80 may control sensing module 86 and therapy module 84 accordingto additional settings not described herein in order to ensure properoperation of IMD 26 during an MRI scan.

In some examples, field discrimination module 89 may include settingsfor enabling portions the field discrimination functionality. Forexample, field discrimination module 89 may enable torque sensors 58,e.g., supply current to coil 66 of torque sensors 58 or provide power toany active components of torque sensors 58 (such as force sensors 68 ininstances in which force sensors 68 are active sensors), in response tothe output of field strength sensor 60. In particular, fielddiscrimination module 89 provides power to torque sensor in response todetecting a magnetic field with a strength that exceeds a minimumthreshold. The minimum threshold may be a value indicating a minimummagnetic field strength which control module 56 may identify as eithertelemetry head field or as static MRI field. When the detected magneticfield is weaker than the lower threshold, control module 56 may operateIMD 26 in the normal mode. The lower threshold value may be set to avalue that reliably indicates that IMD 26 is exposed to a magneticfield, such as a reliable indication that telemetry head magnet 46 isnear to IMD 26 or that MRI device 16 is near to IMD 26. In other words,the lower threshold value may be set so that control module 56 ignoresmagnetic fields that are weaker than may be indicative of telemetry headmagnet 46 or MRI device 16. The lower threshold value may be programmedsuch that control module 56 rejects “noise” or magnetic fields producedby sources other than telemetry head magnet 46 or MRI device 16. In someexamples, the lower threshold may be set to approximately 1-2 mT. Inthis manner, when no magnetic field that exceeds the minimum thresholdis detected, no current is supplied to torque sensor and no power isprovided to any components of torque sensors 58, thereby conservingpower resources of IMD 26. When a magnetic field that exceeds thethreshold is detected, torque sensors 58 may be enabled to measure thetorque imposed on coil 66.

Field discrimination module 89 may, for example, enable torque sensors58 by providing any necessary power to components of torque sensors 58.Field discrimination module 89 may also enable torque sensors 58 byproviding the current to coil 66 to generate the internal magneticfield. In one example, the current is continuously supplied to coil 66when torque sensors 58 is enabled. In other examples, fielddiscrimination module 89 may duty cycle the current provided to coil 66in order to further conserve power. For instance, field discriminationmodule 89 may provide current to coil 66 every few seconds.

In some instances, processing module 80 may operate IMD 26 in a genericmagnet mode in response to the magnitude of the magnetic field exceedingthe minimum threshold and then transition to the MRI mode operate IMD 26in the MRI mode when the source is identified as the primary magnet ofMRI device 16 or the telemetry head mode when the source is identifiedas telemetry head magnet 46 based on the output of torque sensor, asdescribed in detail herein. In one example, the generic magnet mode maybe the same as the telemetry head mode.

In some examples, processing module 80 may be configured to indicate,via communication module 88, to an external computing device when thestatic MRI field is detected. For example, an external computing devicemay include programmer 22, or any other computing device within theimaging room in which the MRI device is located. Upon detection of thestatic MRI field, processing module 80 may indicate, via communicationmodule 88, to the external computing device that the patient has an IMDthat is capable of detecting the static MRI field and/or that the staticMRI field is detected. The external computing device may then display anindicator to a clinician, e.g., on a display, that IMD 26 has detectedthe MRI device and is prepared for the MRI scan.

As a further example, upon detection of the static MRI field, processingmodule 80 may indicate, via communication module 88, to the externalcomputing device that the static MRI field is detected. The externalcomputing device may then send an acknowledgement to IMD 26 in responseto the indication received from communication module 88. In response toreceipt of the acknowledgement, processor 80 may operate IMD 26 in theMRI mode.

Although IMD 26 is described above as having one or more magnetic fieldtorque sensor 58 described herein, the techniques described herein arenot limited to use of such a torque sensor. Any sensor capable ofdetecting a torque caused by entering an environment with a large staticmagnetic field may be used instead of the specific torque sensordescribed herein.

FIG. 7 is a flow diagram illustrating an example method of operation ofan IMD including a torque sensor in accordance with this disclosure.Initially, field discrimination module 89 supplies a current to coils 66of torque sensors 58 (90). The amplitude of the current supplied tocoils 66 may be selected to provide torque sensors 58 with the desiredsensitivity. In other instances, other components of control module 56may supply the current to coils 66 of torque sensors 58.

Field discrimination module 89 obtains signals output by force sensors68 representative of the force detected by force sensors 68 (92). Asdescribed in detail above, force sensors 58 may be aligned on oppositesides of coils 66 such that there are two force sensors on each side ofcoils 66 that sandwich coils 66. When patient 10 and IMD 26 aresubjected to an external magnetic field, the external magnetic fieldinteracts with an internal magnetic field generated by the currentthrough the loop thereby imposing a torque on coils 66 in an attempt toalign a magnetic moment of coils 66 with the external magnetic field.Force sensors 68 of torque sensors 58 generate signals representative ofthe force imposed on them, which are then obtained by fielddiscrimination module 89.

Field discrimination module 89 computes a force detected on each offorce sensors 68 by the torque on coils 66 caused by the externalmagnetic field using the signals obtained from force sensors 68 (94).Field discrimination module 89 detects whether forces exist on forcesensors 68 on opposing sides of either of coils 66 and in opposingdirections (95). For example, field discrimination module 89 may detectwhether forces are detected on force sensors 68A and 68D of torquesensor 68 of FIGS. 4A and 4B or forces that exceed a threshold aredetected on force sensors 68B and 68C of torque sensor 68 of FIGS. 4Aand 4B. When forces do not exist on force sensors 68 on opposing sidesof either of coils 66 and in opposing directions, field discriminationmodule 89 determines that patient 10 and IMD 26 are not in the presenceof the static MRI field and continues to provide current to coil oftorque sensor 58 (90).

When forces do not exist on force sensors 68 on opposing sides of eitherof coils 66 and in opposing directions, field discrimination module 89,field discrimination module 89 determines whether the force on the forcedetected on force sensors 68 on opposing sides of either of coils 66 andin opposing directions exceeds a threshold value (96). When the forcesdo not exceed the threshold value, field discrimination module 89determines that patient 10 and IMD 26 are not in the presence of thestatic MRI field. When the forces do exceed the threshold value, fielddiscrimination module 89 determines that patient 10 and IMD 26 are inthe presence of the static MRI field (98). Processing module 80transitions IMD 26 from operation in the normal mode to operation in theMRI mode in response detecting the presence of the static MRI field ofMRI device 16 (99).

FIG. 8 is a flow diagram illustrating an example method of operation ofan IMD in accordance with this disclosure. Initially, fielddiscrimination module 89 obtains signals output by magnetic fieldstrength sensor 60 representative of a magnitude of a magnetic field towhich IMD 26 exposed (100). Field discrimination module 89 determineswhether the magnitude of the magnetic field is greater than a minimumthreshold (102). If the magnitude is not greater than the minimumthreshold, field discrimination module 89 continues to monitor theoutput of field strength sensor 60.

If the magnitude is not greater than the minimum threshold, fielddiscrimination module 89 enables torque sensors 58 (104). In the case ofthe example torque sensors 58 described above with respect to FIGS. 4A,4B, 5A, and 5B, field discrimination module 89 may enable torque sensor58 by supplying a current to coils 66 of torque sensors 58. In otherexamples, field discrimination module 89 may also provide power to othercomponents such as active sensor components used to detect the torqueexerted by and external magnetic field.

Field discrimination module 89 obtains signals output by torque sensors58 (106). In the example torque sensors 58 described above with respectto FIGS. 4A, 4B, 5A, and 5B, force sensors 68 may be aligned on oppositesides of coils 66 such that there are two sensors on each side of coils66 that sandwich coils 66 and output signals representative of the forcedetected by force sensors 68. When patient 10 and IMD 26 are subjectedto an external magnetic field, the external magnetic field interactswith an internal magnetic field generated by the current through theloop thereby imposing a torque on coils 66 in an attempt to align amagnetic moment of coils 66 with the external magnetic field. Forcesensors 68 of torque sensors 58 generate signals representative of theforce imposed on them, which are then obtained by field discriminationmodule 89. Other torque sensors may output other indications of torque.

Field discrimination module 89 determines whether a detected torqueexceeds a threshold torque value (110). For the example torque sensors58 described above with respect to FIGS. 4A, 4B, 5A, and 5B, fielddiscrimination module 89 may determine whether a detected torque exceedsa threshold torque value using the techniques described with respect toblocks 94-96 of FIG. 7. However, other techniques for determiningwhether the torque exceeds a threshold may be utilized depending on thetype of torque sensor used.

When the detected torque does not exceed the threshold torque value,field discrimination module 89 determines that patient 10 and IMD 26 arein the presence of telemetry head magnet 46 (112). Processing module 80transitions IMD 26 from operation in the normal mode to operation in thetelemetry head mode in response detecting the presence of the telemetryhead magnet 46 (114).

When the detected torque exceeds the threshold torque value, fielddiscrimination module 89 determines that patient 10 and IMD 26 are inthe presence of the static MRI field (116). Processing module 80transitions IMD 26 from operation in the normal mode to operation in theMRI mode in response detecting the presence of the static MRI field ofMRI device 16 (118). In this manner, IMD 26 may utilize torque sensors58 to differentiate between the telemetry head magnet and the primarymagnet of MRI device 16. Additionally, by only enabling torque sensor 56when a magnetic field is detected using the magnetic field strengthsensor 60, IMD 26 conserves energy by only needing to supply a currentwhen a magnetic field is present.

FIG. 9 is a flow diagram illustrating another example method ofoperation of an IMD in accordance with this disclosure. Fielddiscrimination module 89 obtains signals output by torque sensors 58(120). In the example torque sensors 58 described above with respect toFIGS. 4A, 4B, 5A, and 5B, force sensors 68 may be aligned on oppositesides of coils 66 such that there are two sensors on each side of coils66 that sandwich coils 66 and output signals representative of the forcedetected by force sensors 68. When patient 10 and IMD 26 are subjectedto an external magnetic field, the external magnetic field interactswith an internal magnetic field generated by the current through theloop thereby imposing a torque on coils 66 in an attempt to align amagnetic moment of coils 66 with the external magnetic field. Forcesensors 68 of torque sensors 58 generate signals representative of theforce imposed on them, which are then obtained by field discriminationmodule 89. Other torque sensors may output other indications of torque.

Field discrimination module 89 determines whether a detected torqueexceeds a threshold torque value (122). For the example torque sensors58 described above with respect to FIGS. 4A, 4B, 5A, and 5B, fielddiscrimination module 89 may determine whether a detected torque exceedsa threshold torque value using the techniques described with respect toblocks 94-96 of FIG. 7. However, other techniques for determiningwhether the torque exceeds a threshold may be utilized depending on thetype of torque sensor used. When the detected torque does not exceed thethreshold torque value, field discrimination module 89 determines thatpatient 10 and IMD 26 are not exposed to the static MRI field andcontinues to obtain signals output by torque sensors 58 (120).

When the detected torque exceeds the threshold torque value, fielddiscrimination module 89 determines that patient 10 and IMD 26 are inthe presence of the static MRI field (124). Processing module 80transitions IMD 26 from operation in the normal mode to operation in theMRI mode in response detecting the presence of the static MRI field ofMRI device 16 (126). In this manner, torque sensors 58 may be used as amechanism to detect the primary magnet of MRI device 16 by setting thethresholds appropriately.

Although FIGS. 6-9 are described in the context of torque sensors 58,control module 56 may obtain and analyze signals from torque sensor 58′or any other torque sensor. Various examples have been described. Theseand other examples are within the scope of the following claims.

1. A sensor comprising: a conductive coil forming a loop having one ormore turns; a first sensing element adjacent to a first portion of theconductive coil and configured to generate an output that changes as afunction of a force imposed on the first sensing element by the firstportion of the conductive coil; a second sensing element adjacent to thefirst portion of the conductive coil and configured to generate anoutput that changes as a function of a force imposed on the secondsensing element by the first portion of the conductive coil, wherein thefirst portion of the conductive coil is located between the firstsensing element and the second sensing element; a third sensing elementadjacent to a second portion of the conductive coil and configured togenerate an output that changes as a function of a force imposed on thethird sensing element by the second portion of the conductive coil,wherein the second portion of the conductive coil is located on theopposite side of the loop as the first portion of the conductive coil;and a fourth sensing element adjacent to the second portion of theconductive coil and configured to generate an output that changes as afunction of a force imposed on the fourth sensing element by the secondportion of the conductive coil, wherein the second portion of theconductive coil is located between the third sensing element and thefourth sensing element.
 2. The sensor of claim 1, wherein the conductivecoil comprises a first conductive coil, the sensor further comprising: asecond conductive coil forming a second loop having one or more turns,the second conductive coil orthogonal to the first conductive coil; afifth sensing element adjacent to a first portion of the secondconductive coil and configured to generate an output that changes as afunction of a force imposed on the fifth sensing element by the firstportion of the second conductive coil; a sixth sensing element adjacentto the first portion of the second conductive coil and configured togenerate an output that changes as a function of a force imposed on thesixth sensing element by the first portion of the second conductivecoil, wherein the first portion of the second conductive coil is locatedbetween the fifth sensing element and the sixth sensing element; aseventh sensing element adjacent to a second portion of the secondconductive coil and configured to generate an output that changes as afunction of a force imposed on the seventh sensing element by the secondportion of the second conductive coil, wherein the second portion of thesecond conductive coil is located on the opposite side of the secondloop than the first portion of the second conductive coil; and an eighthsensing element adjacent to the second portion of the second conductivecoil and configured to generate an output that changes as a function ofa force imposed on the eighth sensing element by the second portion ofthe second conductive coil, wherein the second portion of the secondconductive coil is located between the seventh sensing element and theeighth sensing element.
 3. The sensor of claim 2, further comprising ahousing enclosing the first and second conductive coils and the first,second, third, fourth, fifth, six, seventh and eighth sensing elements.4. The sensor of claim 2, further comprising: a first housing enclosingthe first conductive coil and the first, second, third, and fourthsensing elements; and a second housing enclosing the second conductivecoil and the fifth, six, seventh and eighth sensing elements.
 5. Thesensor of claim 1, wherein the sensing elements comprise piezoelectricsensing elements.
 6. The sensor of claim 1, wherein the first and secondportions of the conductive coil reside along a first axis of the loop,the sensor further comprising: a ninth sensing element adjacent to athird portion of the conductive coil and configured to generate anoutput that changes as a function of a force imposed on the ninthsensing element by the third portion of the conductive coil; a tenthsensing element adjacent to the third portion of the conductive coil andconfigured to generate an output that changes as a function of a forceimposed on the tenth sensing element by the third portion of theconductive coil, wherein the third portion of the conductive coil islocated between the tenth sensing element and the eleventh sensingelement; an eleventh sensing element adjacent to a fourth portion of theconductive coil and configured to generate an output that changes as afunction of a force imposed on the eleventh sensing element by thefourth portion of the conductive coil, wherein the fourth portion of theconductive coil is located on the opposite side of the loop as the thirdportion of the conductive coil; and a twelfth sensing element adjacentto the fourth portion of the conductive coil and configured to generatean output that changes as a function of a force imposed on the twelfthsensing element by the fourth portion of the conductive coil, whereinthe fourth portion of the conductive coil is located between theeleventh sensing element and the twelfth sensing element, wherein thethird and fourth portions of the conductive coil reside along a secondaxis of the loop.
 7. (canceled)
 8. An implantable medical devicecomprising: a torque sensor that includes: a conductive coil forming aloop having one or more turns; a first sensing element adjacent to afirst portion of the conductive coil and configured to generate anoutput that changes as a function of a force imposed on the firstsensing element by the first portion of the conductive coil; a secondsensing element adjacent to the first portion of the conductive coil andconfigured to generate an output that changes as a function of a forceimposed on the second sensing element by the first portion of theconductive coil, wherein the first portion of the conductive coil islocated between the first sensing element and the second sensingelement; a third sensing element adjacent to a second portion of theconductive coil and configured to generate an output that changes as afunction of a force imposed on the third sensing element by the secondportion of the conductive coil, wherein the second portion of theconductive coil is located on the opposite side of the loop as the firstportion of the conductive coil; and a fourth sensing element adjacent tothe second portion of the conductive coil and configured to generate anoutput that changes as a function of a force imposed on the fourthsensing element by the second portion of the conductive coil, whereinthe second portion of the conductive coil is located between the thirdsensing element and the fourth sensing element; and a control moduleconfigured to analyze the output of the torque sensor to detect thepresence of an external magnetic field and control operation of theimplantable medical device based on the analysis.
 9. The implantablemedical device of claim 8, wherein the control module is furtherconfigured to transition operation of the implantable medical device toa magnetic resonance imaging (MRI) operating mode in response todetecting the presence of the external magnetic field.
 10. Theimplantable medical device of claim 8, wherein the control module isfurther configured to supply a current to the conductive coil of thetorque sensor to generate an internal magnetic field that interacts withthe external magnetic field to produce the torque imposed on theconductive coil.
 11. The implantable medical device of claim 10, whereinthe control module periodically supplies the current to the conductivecoil in accordance with a predefined duty cycle.
 12. The implantablemedical device of claim 10, further comprising a magnetic field strengthsensor separate from the torque sensor, the magnetic field strengthsensor configured to output a signal that varies as a function of thestrength of the external magnetic field, wherein the control modulesupplies the current to the conductive coil of the torque sensor inresponse to detecting that the strength of the external magnetic fieldexceeds a strength threshold.
 13. The implantable medical device ofclaim 12, wherein the control module is configured to determine if atorque imposed on the conductive coil exceeds a threshold torque value,detect presence of a static magnetic resonance imaging (MRI) field whenthe torque exceeds the threshold torque value, and transition operationof the implantable medical device to an MRI operating mode in responseto detecting the presence of the external magnetic field.
 14. Theimplantable medical device of claim 13, wherein the control module isconfigured to detect presence of a telemetry head magnet when the torqueimposed on the conductive coil does not exceed the threshold torquevalue and transition operation of the implantable medical device to atelemetry head operating mode in response to detecting the presence ofthe external magnetic field.
 15. The implantable medical device of claim13, wherein the control module is configured to determine that thetorque imposed on the conductive coil exceeds a threshold torque valuewhen a first force on the first sensing element and a second force onthe third sensing element are detected at substantially the same timeand that each of the first force and the second force exceeds athreshold force value.
 16. The implantable medical device of claim 8,wherein the conductive coil of the torque sensor comprises a firstconductive coil, the torque sensor further comprising: a secondconductive coil forming a second loop having one or more turns, thesecond conductive coil orthogonal to the first conductive coil; a fifthsensing element adjacent to a first portion of the second conductivecoil and configured to generate an output that changes as a function ofa force imposed on the fifth sensing element by the first portion of thesecond conductive coil; a sixth sensing element adjacent to the firstportion of the second conductive coil and configured to generate anoutput that changes as a function of a force imposed on the sixthsensing element by the first portion of the second conductive coil,wherein the first portion of the second conductive coil is locatedbetween the fifth sensing element and the sixth sensing element; aseventh sensing element adjacent to a second portion of the secondconductive coil and configured to generate an output that changes as afunction of a force imposed on the seventh sensing element by the secondportion of the second conductive coil, wherein the second portion of thesecond conductive coil is located on the opposite side of the secondloop than the first portion of the second conductive coil; and an eighthsensing element adjacent to the second portion of the second conductivecoil and configured to generate an output that changes as a function ofa force imposed on the eighth sensing element by the second portion ofthe second conductive coil, wherein the second portion of the secondconductive coil is located between the seventh sensing element and theeighth sensing element.
 17. The implantable medical device of claim 16,wherein the torque sensor further comprises a housing enclosing thefirst and second conductive coils and the first, second, third, fourth,fifth, six, seventh and eighth sensing elements.
 18. The implantablemedical device of claim 16, wherein the torque sensor further comprises:a first housing enclosing the first conductive coil and the first,second, third, and fourth sensing elements; and a second housingenclosing the second conductive coil and the fifth, six, seventh andeighth sensing elements.
 19. The implantable medical device of claim 8,wherein the first and second portions of the conductive coil of thetorque sensor reside along a first axis of the loop, the torque sensorfurther comprising: a ninth sensing element adjacent to a third portionof the conductive coil and configured to generate an output that changesas a function of a force imposed on the ninth sensing element by thethird portion of the conductive coil; a tenth sensing element adjacentto the third portion of the conductive coil and configured to generatean output that changes as a function of a force imposed on the tenthsensing element by the third portion of the conductive coil, wherein thethird portion of the conductive coil is located between the tenthsensing element and the eleventh sensing element; an eleventh sensingelement adjacent to a fourth portion of the conductive coil andconfigured to generate an output that changes as a function of a forceimposed on the eleventh sensing element by the fourth portion of theconductive coil, wherein the fourth portion of the conductive coil islocated on the opposite side of the loop as the third portion of theconductive coil; and a twelfth sensing element adjacent to the fourthportion of the conductive coil and configured to generate an output thatchanges as a function of a force imposed on the twelfth sensing elementby the fourth portion of the conductive coil, wherein the fourth portionof the conductive coil is located between the eleventh sensing elementand the twelfth sensing element, wherein the third and fourth portionsof the conductive coil reside along a second axis of the loop.
 20. Theimplantable medical device of claim 8, further comprising a therapymodule configured to deliver electrical stimulation therapy to apatient, wherein the control module adjusts operation of the therapymodule based on the detection.
 21. An implantable medical systemcomprising: at least one implantable medical lead that includes one ormore electrodes; and an implantable medical device coupled to the atleast one implantable medical lead and configured to transmit a therapyvia the one or more electrodes of the at least one implantable medicallead, the implantable medical device including: a torque sensor thatincludes: a conductive coil forming a loop having one or more turns; afirst sensing element adjacent to a first portion of the conductive coiland configured to generate an output that changes as a function of aforce imposed on the first sensing element by the first portion of theconductive coil; a second sensing element adjacent to the first portionof the conductive coil and configured to generate an output that changesas a function of a force imposed on the second sensing element by thefirst portion of the conductive coil, wherein the first portion of theconductive coil is located between the first sensing element and thesecond sensing element; a third sensing element adjacent to a secondportion of the conductive coil and configured to generate an output thatchanges as a function of a force imposed on the third sensing element bythe second portion of the conductive coil, wherein the second portion ofthe conductive coil is located on the opposite side of the loop as thefirst portion of the conductive coil; and a fourth sensing elementadjacent to the second portion of the conductive coil and configured togenerate an output that changes as a function of a force imposed on thefourth sensing element by the second portion of the conductive coil,wherein the second portion of the conductive coil is located between thethird sensing element and the fourth sensing element; a control moduleconfigured to analyze the output of the torque sensor to detect thepresence of an external magnetic field; and a therapy module configuredto deliver electrical stimulation therapy to a patient, wherein thecontrol module adjusts operation of the therapy module based on thedetection.